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Morphology-Mediated Photoresponsive and Fluorescence Behaviors of Azobenzene-Containing Block Copolymers Pin-Chi Huang, Jitendra Mata, Chun-Ming Wu, and Chieh-Tsung Lo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01033 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Morphology-Mediated Photoresponsive and Fluorescence Behaviors of Azobenzene-Containing Block Copolymers Pin-Chi Huang,a Jitendra P. Mata,b Chun-Ming Wu,c Chieh-Tsung Loa* a

b

Department of Chemical Engineering, National Cheng Kung University, Tainan City 701, Taiwan Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology

Organization, Lucas Heights, NSW 2234, Australia c

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park,

Hsinchu 30076, Taiwan

*Corresponding author Postal address: Department of Chemical Engineering, National Cheng Kung University, Tainan City 701, Taiwan Tel.: +886-6-2757575 ext. 62647 E-mail address: [email protected]

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ACS Paragon Plus Environment

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Abstract We investigated the relationship between the self-assembled morphology of poly(tert-butyl acrylate)-block-poly(6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-b-PAzoMA) block copolymers and their photoresponsive and fluorescence behaviors. The morphology of PtBA-b-PAzoMA copolymers was manipulated by dissolving them in mixed dimethylformamide (DMF)/hexanol solvents. When PtBA-b-PAzoMA was dissolved in DMF-rich (neutral) solvents, a favorable interaction between the DMF molecules and both blocks resulted in a random-coiled conformation. The unconfined morphology facilitated the formation of both nonassociated and head-to-head organized azobenzene mesogens, which promoted fluorescence emission. When hexanol, a PtBA-selective solvent, was added to DMF, the solvency of PtBA-b-PAzoMA worsened, leading to its assembly into micelles, with PAzoMA in the micelle core. The confinement of azobenzene moieties in the micelle core hindered their trans-to-cis photoisomerization, thereby considerably decreasing the kinetics of photoisomerization and the population of cis isomers. Additionally, a nanoconfined geometry resulted in compactly packed chromophores, causing fluorescence loss. When PtBA-b-PAzoMA was exposed to UV light, the increased number of cis isomers hampered the closely packed mesogens, resulting in a substantial enhancement of fluorescence emission. When the mole fraction of the PAzoMA block was increased, PtBA-b-PAzoMA formed clusters, causing the slow kinetics of photoisomerization and fluorescence quenching.

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Introduction Photoresponsive polymers composed of azobenzenes and other chromophores are an important class of materials because they exhibit unique characteristics, such as photoisomerization, photochemical phase transitions, photoalignment, and photoinduced cooperative motions.1 Their photoresponsive behaviors depend strongly on the organization of chromophores in the polymer matrix. To effectively manipulate their photoresponsive behaviors, chromophores can be incorporated as one of the constituents of block copolymers. Block copolymers form microphase-separated morphologies on a nanometer scale. A nanoconfined geometry can facilitate the self-arrangement of mesogens into periodical ordering, forming hierarchical structures.2-5 Furthermore, non-photoresponsive segments can be designed to impart additional properties to block copolymers, such as hydrophilicity, pH sensitivity, flexibility, and crystallization. These multifunctional block copolymers offer potential opportunities in versatile applications. Photoisomerization of various azobenzene-containing block copolymers, which is one of the most unique properties of azobenzene derivatives, has been studied extensively.6-10 Such photoisomerization results from the conversion of azobenzene mesogens from trans isomers to cis isomers when exposed to UV light. The reverse cis-to-trans isomerization can be driven by visible light or by incubation in the dark. The photoresponsive behavior of these block copolymers is mainly controlled by their morphology. Su et al. reported a morphological change from spherical to disk-like, micron-sized particles when an amphiphilic block copolymer containing poly(acrylic acid) and azobenzenes (PAA-b-PAzoMA) dissolved in a mixed tetrahydrofuran (THF)/water solvent was 3


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illuminated with UV light.8 The morphological change was accompanied by the disruption of aggregated mesogens, which resulted in the formation of isolated mesogens. Additionally, the degree of isomerization of PAA-b-PAzoMA in the mixed THF/water solvent was much less than that of PAA-b-PAzoMA in a neutral (THF) solvent. Similarly, Liu and Chiu synthesized block copolymers composed of poly(methyl methacrylate) and pendant azobenzene segments (PMMA-b-PAzoMA).11 UV irradiation of PMMA-b-PAzoMA dissolved in a mixed THF/water solvent induced a morphological change from spherical micelles to rod-like particles. These various morphologies result from the self-assembly of azobenzene-containing block copolymers in solvents, in which solvent molecules exhibit preferential affinity to one of the segments, forming a micellar structure. The self-assembled structures provide a confined environment, thereby hindering trans-to-cis

photoisomerization.

Consequently,

the

kinetics

of

photoisomerization

of

azobenzene-containing block copolymers in selective solvents were lower than those of photoisomerization of the block copolymers in a neutral solvent.9,10 Furthermore, such confinement causes the self-organization of azobenzene mesogens, forming side-by-side (H-type) and head-to-head (J-type) aggregates. These aggregates result in the blueshift and redshift of the absorption maximum, respectively, when compared with the absorption maximum of nonassociated mesogens.6 In addition to solvency, pH values influence the photoresponsive behavior of azobenzene-containing block copolymers.12,13 By tailoring a pH-sensitive segment in block copolymers, the dissociation/formation of micelles can be leveraged by varying pH values, leading to a change in the kinetics of photoisomerization. 4


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Another unique feature of azobenzene derivatives is their fluorescence emission ability. However, the fluorescence of azobenzene derivatives is hardly detected because the nonradiative relaxation process inhibits their fluorescence emission.14 Azobenzene mesogens aggregated in a confined space into conformations such as bilayers,15,16 micelles,12,13,17,18 and vesicles19,20 have been reported to be exceptions. This aggregation-induced emission results from closely packed chromophores in confined geometries that inhibit the efficient relaxation process, thereby enabling fluorescence. Using azobenzene-containing block copolymers consisting of a hydrophilic quaternized poly(4-vinyl pyridine) block as a model system, Bo and Zhao demonstrated that these block copolymers were nonfluorescent when dissolved in dimethylformamide (DMF), which is a good solvent for both blocks.12 Upon the addition of water, micellization occurred, resulting in the confinement of azobenzene groups in the micelle core. Furthermore, fluorescence emission increased monotonically with the addition of water. Xu et al. investigated the fluorescence behavior of

amphiphilic

block

copolymers

poly{6-(4phenylazophenoxy)hexyl

methacrylate}-block-poly{2-(dimethylamino)ethyl methacrylate} (PAHMA-b-PDMAEMA).17 The fluorescence of PAHMA-b-PDMAEMA in chloroform was considerably affected by the UV irradiation time: the fluorescence intensity increased with short exposure (a few minutes) and subsequently decreased with prolonged exposure. The changes in fluorescence emission were attributed to the structural change of PAHMA-b-PDMAEMA in chloroform from nanometer-scale aggregates to large aggregates; this structural change was caused by the interaction between cis isomers during trans-to-cis photoisomerization. Similarly, the monotonic reduction of the 5


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fluorescence intensity with the UV irradiation time was observed for poly(ethylene glycol) (PEG)-based azobenzene-containing diblock copolymers in a mixed THF/water solvent.18 In contrast to the results of Xu et al.,17 UV irradiation of PEG-based block copolymers caused a decrease in the micellar size, thereby decreasing the amount of excited azobenzenes available for radiative relaxation. Thus, fluorescence emission was quenched. Interestingly, when the PEG-based block copolymer was dissolved in THF, fluorescence enhancement with the UV irradiation time was observed. Li et al. suggested that photoisomerization-induced fluorescence emission was not caused by the formation of aggregates.19 Instead, the presence of bent-shaped cis isomers hampered the effective conjugation of electrons on azobenzene moieties with those in indazole groups, thereby intensifying fluorescence. The aforementioned reports demonstrate that azobenzene-containing block copolymers emit fluorescence under some conditions and suggest the plausible mechanisms of fluorescence emission. However, some results are not entirely consistent; therefore, additional studies are needed to clarify the mechanism of fluorescence emission in azobenzene-containing block copolymers. As indicated previously, the self-assembled morphology of azobenzene-containing block copolymers is the most crucial parameter that influences their photoresponsive and fluorescence behaviors. In this study, we utilized small-angle neutron scattering (SANS) to probe the morphology of poly(tert-butyl acrylate)-block-poly(6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-b-PAzoMA) block copolymers in mixed DMF/hexanol solvents. DMF is a neutral solvent for both blocks, whereas hexanol is a PtBA-selective solvent. By varying the 6


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hexanol amount in mixed solvents, we were able to leverage the solvency of the PtBA-b-PAzoMA solution and subsequently manipulate the morphology of PtBA-b-PAzoMA. Unlike transmission electron microscopy, which has been used to characterize the structure of azobenzene-containing block copolymers in most studies,8-13,17-21 SANS provides more detailed and statistical structural information that is essential to understand the photoresponsive and fluorescence behaviors of these block copolymers in their natural environment. In this paper, we present a generalized experimental model of the relationship between the morphologies of these block copolymers and their photoresponsive and fluorescence properties. The valuable data provided in this study can complement the literature and provide insights into the mechanism of the photoresponsive and fluorescence properties of azobenzene-containing derivatives.

Experimental Section Materials All purchased chemicals were used as received without further purification. p-Anisidine (99%), phenol

(99%),

6-chlorehexanol

(97%),

methacryloyl

chloride

(97%),

N,N,N′N′N′′-pentamethyldiethylenetriamine (PMDETA, 98%), and hexanol (99%) were provided by Alfa Aesar. Sodium nitrite (NaNO2, 99%), potassium iodide (KI), triethylamine (TEA, 99.9%), calcium hydride (90%), ethyl 2-bromopropionate (2-EBP, 99%), copper bromide (CuBr, 99.999%), and deuterated DMF (d-DMF) were purchased from Sigma-Aldrich. tert-Butyl acrylate (tBA, 99%) and deuterated hexanol (d-hexanol) were obtained from Acros Organics and CDN Isotopes, 7


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respectively. Hydrochloric acid (HCl, 36.5%–38%), potassium carbonate (K2CO3), DMF (99.8%), magnesium sulfate anhydrous (99.8%), acetic acid (100%), and ether (99%) were obtained from J. T. Baker.

Synthesis of macroinitiator In this study, bromide-terminated poly(tert-butyl acrylate) (PtBA-Br) was applied as a macroinitiator. The synthesis reaction used tBA as a monomer, 2-EBP as an initiator, and CuBr complexed with PMDETA as a catalyst22 and was performed at 60 °C for 24 h. After the reaction, the CuBr–PMDETA complex was removed by passing the mixture through a column of aluminum oxide. Subsequently, 500 mL of a mixed methanol/water solution (volume ratio of 1:1) was used to precipitate PtBA-Br. The as-prepared PtBA-Br exhibited number-averaged molecular weights (Mn) of 36300, 17200, and 10600 g/mol, with polydispersity indexes (PDIs, Mw/Mn, where Mw is the weight-averaged molecular weight) of 1.09, 1.18, and 1.08, respectively.

Synthesis of 6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate monomer The

synthesis

approach

for

the

azobenzene-containing

monomer

6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate (AzoMA) has been described in the literature.23,24 Briefly, diazotation of p-anisidine was performed in a mixed solution containing HCl, NaNO2,

and

phenol

at

room

temperature

for

2

h.

Subsequently,

the

resultant

4-methoxy-4′-hydroxyazobenzene was mixed with K2CO3 and KI in DMF, and 6-chlorohexanol 8


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was added to the mixture for the synthesis of 4-methoxy-4′-(6-hydroxyhexyloxy)azobenzene. Further reaction of 4-methoxy-4′-(6-hydroxyhexyloxy)azobenzene with TEA and methacryloyl chloride resulted in the formation of AzoMA. The characterizations of azobenzene-containing monomers are summarized in the Supporting Information (Figures S1 and S2).

Synthesis of PtBA-b-PAzoMA block copolymers To synthesize PtBA-b-PAzoMA block copolymers, atom transfer radical polymerization (ATRP) was performed using AzoMA and PtBA-Br as the monomer and macroinitiator, respectively.21 Initially, PtBA-Br, AzoMA, and the CuBr–PMDETA complex were dissolved in THF, and the reaction proceeded at 60 °C for 14 h. After the reaction, the purification of PtBA-b-PAzoMA was performed by passing the resultant mixture through a column of aluminum oxide. PtBA-b-PAzoMA was then precipitated with methanol.

Characterization The 1H nuclear magnetic resonance (NMR) spectra of PtBA-Br, azobenzene-containing monomers, and block copolymers were recorded using a Bruker Avance 600 spectrometer. Solutions for NMR measurements were prepared by dissolving these materials in CDCl3. The molecular weight and molecular weight distribution of PtBA-Br and PtBA-b-PAzoMA were determined using a gel permeation chromatography (GPC) instrument (Lab Alliance RI2000), which was equipped with a refractive index detector (Schambeck SFD GmbH) and a column (PL 9


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gel 5 µm mixed-D). THF was used as the elution solvent, with an elution rate of 1 mL/min. The instrument was calibrated using a polystyrene standard. SANS experiments were conducted using the Quokka beamline at the Australian Nuclear Science and Technology Organisation.25,26 For SANS measurements, PtBA-b-PAzoMA block copolymers dissolved in mixed deuterated solvents (d-DMF/d-hexanol = 100/0, 90/10, 50/50, and 0/100 [vol/vol]) were sealed in quartz cells with a 1-mm path length. After the measurements, isotropic scattering patterns were radially averaged to reduce them to one-dimensional intensity profiles in the Q range of 0.007–0.54 Å−1, where Q = 4π sinθ/λ, 2θ is the scattering angle, and λ is the wavelength of neutrons (5 Å). The scattering data were corrected for the empty cell, sample transmission, and solvent scattering to produce the absolute scale in a unit of cm−1. The morphology of PtBA-b-PAzoMA in solvents were characterized using transmission electron microscopy (TEM), operated on a Hitachi H7500 electron microscope at 80 keV. Samples were prepared by leaving a drop of the polymer solutions on a copper grid coated with carbon film. The specimens were then stained with RuO4 for 30 min. The UV-vis spectra of PtBA-b-PAzoMA in mixed DMF/hexanol solvents were recorded using a Perkin Elmer Lamba 25 spectrophotometer. The samples were illuminated by UV light with a wavelength of 365 nm and an intensity of 4.5 mW/cm2. Visible light irradiation was conducted at a wavelength of 430 nm and an intensity of 4.3 mW/cm2. Fluorescence emission of PtBA-b-PAzoMA in mixed DMF/hexanol solvents was obtained using a Hitachi F-4500 spectrophotometer.

Results and Discussion 10


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Characterization of PtBA-b-PAzoMA Figure 1 shows the representative 1H-NMR spectrum of PtBA-b-PAzoMA synthesized through ATRP. The chemical structure of PtBA-b-PAzoMA coexisted with the functional groups of AzoMA (Figure S1) and tBA (Figure S3). Additionally, the characteristic peaks of protons on CH2=CH of azobenzene monomers at chemical shifts of 5.5 and 6.1 ppm disappeared, indicating that AzoMA was copolymerized with tBA, forming PtBA-b-PAzoMA. The Mn of the synthesized PtBA-b-PAzoMA was obtained by calculating the ratio of the peak area of the protons on the backbone of PtBA at a chemical shift of approximately 2.2 ppm to that of the aromatic protons of azobenzene at a chemical shift of approximately 6.9 ppm. Figure S4 presents the GPC curves of various PtBA-b-PAzoMA block copolymers. The synthesis of PtBA-b-PAzoMA was in a controlled way. Table 1 lists the molecular weight and molecular weight distribution of synthesized PtBA-b-PAzoMA with various compositions.

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Figure 1. 1H NMR spectrum of the PtBA81-b-PAzoMA36 block copolymer.

Table 1. Characteristics of PtBA-b-PAzoMA Mn of

Mn of PtBAb

Mn of PAzoMAc

PtBA-b-PAzoMAa

(g/mol)

(g/mol)

Polymer

PDId

fPAzoMAe

(g/mol) PtBA81-b-PAzoMA36

25.0k

10.6k

14.4k

1.20

0.58

PtBA133-b-PAzoMA19

24.7k

17.2k

7.5k

1.19

0.30

PtBA282-b-PAzoMA18

43.4k

36.3k

7.1k

1.18

0.16

a

PtBAm-b-PAzoMAn where m and n are the degree of polymerization of PtBA and PAzoMA, respectively

b

The Mn of PtBA was measured by GPC using a polystyrene standard

c

The Mn of PAzoMA was calculated from 1H NMR

d

PDI was obtained from GPC

e

fPAzoMA: the mole fraction of PAzoMA

Solubility characteristics of PtBA-b-PAzoMA in DMF and hexanol In this study, we investigated the morphology and photoresponsive and fluorescence behaviors 12


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of PtBA-b-PAzoMA in mixed DMF/hexanol solvents. These behaviors strongly rely on the interaction between the solvent molecules and the individual block of PtBA-b-PAzoMA. The polymer–solvent interaction can be predicted using the concept of solubility parameters. Hildebrand and Scott proposed that when nonpolar molecules are mixed, the energy of mixing (∆Emix) is associated with the energies of vaporization of pure components:27 ∆Emix

φ1φ2

2 2  ∆E  ∆Evap , 2   vap ,1   −   = Vm   Vm ,1   Vm, 2    

2

(1)

where φi, ∆Evap,i, and Vm,i are the volume fraction, energy of vaporization, and molar volume of each

component in the mixture, respectively, and Vm is the average molar volume of the mixture.

∆Evap ,i Vm ,i

is denoted as the cohesive energy density of each component, which is equal to the square root of the solubility parameter of the component (δ). The solubility parameter of a molecule can be divided into three components: the dispersion (nonpolar) Hansen solubility parameter (δd), dipole Hansen solubility parameter (δp), and hydrogen-bonding Hansen solubility parameter (δhb), as follows:28

δ 2 = δ d 2 + δ p 2 + δ hb 2

(2)

The individual Hansen solubility parameters can be calculated using a group contribution method29 (the results are summarized in Table 2).

Table 2. Hansen solubility parameters of molecules used in this study Molecule

δd (MPa1/2)

δp (MPa1/2) 13


δhb (MPa1/2)

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PAzoMA

23.0

12.1

4.9

PtBA

15.3

8.4

5.7

DMF

17.4

13.7

11.3

hexanol

15.9

5.8

12.5

To more clearly and precisely determine the solubility of various molecules, we generated a Teas ternary plot, which is composed of the fraction of each of the Hansen solubility parameters that are calculated to determine the total solubility parameter of each molecule (Figure 2). The fraction of each Hansen solubility parameter is obtained using the following expressions:30

fd =

δd δ d + δ p + δ hb

(3)

fp =

δp δ d + δ p + δ hb

(4)

f hb =

δ hb δ d + δ p + δ hb

(5)

where fd, fp, and fhb are the fractions of the individual Hansen solubility parameters associated with dispersion forces, dipole forces, and hydrogen-bonding interactions, respectively. As shown in Figure 2, the position of DMF is close to that of both PtBA and PAzoMA, indicating that DMF is a good solvent for both molecules. By contrast, the solubility of PtBA and PAzoMA in hexanol is lower than that of PtBA and PAzoMA in DMF. Additionally, PtBA is more soluble in hexanol than PAzoMA is, which is justified by the longer distance between the locations of PAzoMA and hexanol in the Teas plot. Furthermore, we dissolved PtBA and the AzoMA monomer in these solvents to verify solubility obtained using the concept of the solubility parameters. Both PtBA and the AzoMA 14


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monomer were soluble in DMF, confirming that DMF is a good solvent for both PtBA and the AzoMA monomer. By contrast, PtBA was soluble in hexanol, but the AzoMA monomer was insoluble in hexanol, suggesting that hexanol is a selective solvent for PtBA-b-PAzoMA.

Figure 2. Teas ternary plot of the solubility parameters of various solvents and polymers.

Characterization of morphology of PtBA-b-PAzoMA in solution using SANS

The morphology of PtBA-b-PAzoMA in a solvent is strongly influenced by solvency. Hence, the morphology of PtBA-b-PAzoMA block copolymers formed in mixed DMF/hexanol solvents is expected to change with the solvent composition, and for SANS analysis, different modeling schemes are required to describe the structural properties of the copolymers. Figure 3 shows the SANS patterns of PtBA-b-PAzoMA in mixed d-DMF/d-hexanol solvents with various compositions. For PtBA282-b-PAzoMA18 and PtBA133-b-PAzoMA19 dissolved in DMF and mixed solvents containing 90% and 50% DMF, the polymer molecules exhibited nonassociated individual unimers dispersed in solution, and the scattering curves could be fitted with a flexible random-coiled 15


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(Gaussian chain) model.31 Although PtBA-b-PAzoMA contained azobenzene moieties, which exhibited a rigid chain structure rather than a coil structure, the scattering profiles could still be approximated by the random-coiled model. This was presumably because azobenzene mesogens were located in the side chains rather than on the backbone of PtBA-b-PAzoMA. The obtained radii of gyration of the block copolymers are listed in Table 3. When the amount of hexanol in mixed solvents was less than 50%, the radius of gyration of PtBA-b-PAzoMA remained constant, indicating that the addition of this amount of hexanol did not markedly alter the solvency of PtBA-b-PAzoMA. Furthermore, the radius of gyration of PtBA282-b-PAzoMA18 was slightly larger than that of PtBA133-b-PAzoMA19, which was because the molecular weight of the coil block in PtBA282-b-PAzoMA18 was higher than that of the coil block in PtBA133-b-PAzoMA19.

(a)

(b)

16


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(c) Figure 3. SANS data of (a) PtBA282-b-PAzoMA18, (b) PtBA133-b-PAzoMA19, and (c) PtBA81-b-PAzoMA36 in mixed DMF/hexanol solvents. The solid lines represent the fits of various models.

Table 3. Fitted parameters for PtBA-b-PAzoMA in mixed d-DMF/d-hexanol solvents d-DMF：d-hexanol

100:0

90:10 a

50:50 a

Rg (Å)

0:100 b

Rg (Å)

Rc (Å)

Rs (Å)c

nd

Ns (1/µm3)e

PtBA282-b-PAzoMA18

71.3

71.1

71.0

188.0

83.3

31.9

1526

PtBA133-b-PAzoMA19

60.5

60.2

60.2

147.2

41.8

31.3

667

PtBA81-b-PAzoMA36

54.1

54.2

ξf = 121.4

a

Radius of gyration of PtBA-b-PAzoMA

b

Radius of micelle core

c

Thickness of micelle corona

d

Association number of micelles

e

Number density of micelles

f

Correlation length for the polymer chains

insoluble

When PtBA282-b-PAzoMA18 and PtBA133-b-PAzoMA19 were dissolved in hexanol, the

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repulsive interaction between PAzoMA chains and hexanol induced the assembly of PAzoMA segments into the micelle core, whereas the PtBA blocks were organized as the micelle corona to prevent direct contact between PAzoMA and hexanol. Many analytical expressions for the form factor of numerous geometrical shapes have been proposed to elucidate the scattering profiles of micelles formed by block copolymers.32-34 Although the rigid characteristics of azobenzene mesogens confined in the micelle core are expected to alter the interfacial curvature between PtBA and PAzoMA blocks, only the form factor of spherical micelles can be used to model the structure of both PtBA282-b-PAzoMA18 and PtBA133-b-PAzoMA19 in hexanol. The best fits of the scattering data resulted in a core radius of 188.0 Å and a shell thickness of 83.3 Å for the micelles formed by PtBA282-b-PAzoMA18, whereas the micelles formed by PtBA133-b-PAzoMA19 had a core radius of 147.2 Å and a shell thickness of 41.8 Å. Other detailed fitting parameters are summarized in Table 3. Figure

4

confirms

the

spherical

micelles

formed

by

PtBA282-b-PAzoMA18

and

PtBA133-b-PAzoMA19 in hexanol The average diameters of micelles obtained by the TEM images were 63.2 and 29.4 nm for PtBA282-b-PAzoMA18 and PtBA133-b-PAzoMA19, respectively, which were qualitatively consistent with the SANS results. Although the molecular weights of the PAzoMA block in the two copolymers were nearly identical, the micelle core formed by PtBA133-b-PAzoMA19 was much smaller than that formed by PtBA282-b-PAzoMA18. This was attributed to the higher fraction of the PAzoMA block in PtBA133-b-PAzoMA19, which caused its poorer miscibility in hexanol, resulting in the formation of a more compact micelle core.

18


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(a)

(b)

Figure 4. TEM images of (a) PtBA282-b-PAzoMA18 and (b) PtBA133-b-PAzoMA19 in hexanol.

Figure S5 shows the TEM images of PtBA282-b-PAzoMA18 and PtBA133-b-PAzoMA19 in hexanol-rich mixed solvents. Similar to these two block copolymers in hexanol, micelles were formed when they were dissolved in the mixed solvents containing 90% and 75% hexanol. The diameters of micelles formed by PtBA282-b-PAzoMA18 in the mixed solvents containing were 90% and 75% hexanol were 44.7 and 38.7 nm, respectively. By contrast, the diameters of micelles formed by PtBA133-b-PAzoMA19 in the mixed solvents containing were 90% and 75% hexanol were 26.4 and 15.6 nm, respectively. The diameters of micelles for both block copolymers decreased monotonically with an increase in the content of DMF. When DMF was added to hexanol, the solvency of PtBA-b-PazoMA increased. This reduced the tendency of PtBA-b-PazoMA assembly, resulting in a decrease in the micelle size. By contrast, PtBA81-b-PAzoMA36 with a high fraction of the PAzoMA block limited the solubility behavior in mixed solvents, resulting in the formation of different structures. When the amount of hexanol in the mixed solvent was considerably low, PtBA81-b-PAzoMA36 was able to 19


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sustain a random-coiled conformation. However, when the amount of hexanol in the mixed solvent was 50%, the random-coiled model failed to match the scattering curve of PtBA81-b-PAzoMA36. Comprehensive analysis of the SANS data of PtBA81-b-PAzoMA36 (Figure 3(c)) revealed that the curve featured an increasing trend at the low-Q regions, indicating the formation of clusters. The structure of PtBA81-b-PAzoMA36 was efficiently predicted using the correlation length model proposed by Hammouda et al.35 The formation of clusters for PtBA81-b-PAzoMA36 in the mixed solvent was attributed to the long chains of the PAzoMA segments, which disallowed full coverage by PtBA chains through micelles. Therefore, PtBA81-b-PAzoMA36 chains aggregated to minimize the repulsive interaction between PAzoMA chains and hexanol. The phase transition from micelles to clusters through the change in the mole fraction of the PAzoMA block (fPAzoMA) has never been reported for azobenzene-containing block copolymers. Figure 5(a) summarizes the phase behavior of PtBA-b-PAzoMA in mixed DMF/hexanol solvents. The morphology of PtBA-b-PAzoMA in various mixed solvents depended strongly on fPAzoMA. When fPAzoMA was low, the solubility of PtBA-b-PAzoMA in mixed solvents was high.

Consequently, PtBA-b-PAzoMA exhibited a random-coiled conformation when dissolved in DMF-rich mixed solvents, whereas PtBA-b-PAzoMA self-assembled into micelles when dissolved in hexanol-rich mixed solvents (Figure 5(b)). By contrast, when fPAzoMA was high, the solubility of PtBA-b-PAzoMA in mixed solvents was low. Additionally, the high fraction of the PAzoMA block hindered the formation of micelles. Consequently, PtBA-b-PAzoMA transformed from the random-coiled conformation in DMF to clusters with an increase in the amount of hexanol in mixed 20


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Langmuir

solvents (Figure 5(b)). Further increasing the amount of hexanol resulted in the insolubility of PtBA-b-PAzoMA in mixed solvents.

(a)

(b) Figure 5. (a) Phase diagram of PtBA-b-PAzoMA in mixed DMF/hexanol solvents as functions of fPAzoMA and solvent composition. Key: (■) random coil; (●) cluster; (▲) micelle; (▼) insolubility. 21


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(b) Schematics of the mechanism of phase transitions.

Photoisomerization of PtBA-b-PAzoMA in mixed solvents

Figure 6 presents the UV-vis spectra of PtBA133-b-PAzoMA19 in DMF. Strong absorbance was observed at the wavelength of 359 nm, which corresponded to the π-π* absorption of azobenzene mesogens with the trans conformation. After the UV irradiation of PtBA133-b-PAzoMA19, absorbance at 359 nm decreased considerably with the exposure time, followed by a substantial increase in absorbance at wavelengths of 311 and 452 nm. These emerging peaks corresponded to the π-π* and n-π* transitions of cis-azobenzene, respectively. The trans-to-cis transition of isomers was completed in 50 s. The content of cis isomers at the photostationary state can be estimated using the following equation:3 cis − isomer (%) =

A0 − A∞ × 100% A0

(6)

where A0 and A∞ are absorbance at the wavelength of 359 nm at the initial and photostationary states, respectively. As shown in Table 4, nearly 95% of trans isomers converted to cis isomers after UV irradiation. Conversely, cis-to-trans photoisomerization was induced by visible light irradiation. As shown in Figure 6(b), visible light irradiation resulted in a substantial increase in the π-π* transition of trans-azobenzene and the progressive reduction of the π-π* and n-π* transitions of cis-azobenzene. At the end of reverse isomerization, the UV spectrum of PtBA133-b-PAzoMA19 in DMF was similar to that of PtBA133-b-PAzoMA19 before UV irradiation, suggesting that the photoisomerization of PtBA-b-PAzoMA was reversible. 22


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(a)

(b)

Figure 6. UV spectra of PtBA133-b-PAzoMA19 in DMF. (a) Irradiated with UV light and (b) recovered with visible light.

Table 4. Characteristics of photoisomerization of different PtBA-b-PAzoMA block copolymers in various mixed solvents obtained through UV spectra DMF:hexanol

PtBA282-b-PAzoMA18

PtBA133-b-PAzoMA19

PtBA81-b-PAzoMA36

40

50

50

95.48

94.82

95.83

360

359

360

50

50

50

96.30

96.52

96.66

λmax (nm)

360

359

360

time (s)a

50

50

50

96.21

94.58

95.25

360

359

360

90

90

90

92.33

82.91

79.32

λmax (nm)

357

343

343

a

120

120

150

88.85

73.95

74.65

λmax (nm)

355

340

342

time (s)a

150

150

--d

a

time (s) 100:0

b

cis (%)

c

λmax (nm) a

time (s) 90:10

b

cis (%)

c

50:50

b

cis (%)

c

λmax (nm) a

time (s) 25:75

b

cis (%)

c

time (s) 10:90

b

cis (%)

c

23


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0:100

cis (%)b

86.83

73.70

--d

λmax (nm)c

355

341

--d

a

Time required to reach the photostationary state

b

Content of cis isomers at the photostationary state

c

Wavelength of maximum absorbance at the initial state

d

″--″ indicates insolubility of the system

When

Page 24 of 42

PtBA133-b-PAzoMA19

was

dissolved

in

mixed

solvents,

both

trans-to-cis

photoisomerization and subsequent cis-to-trans photoisomerization were observed (Figure S6). However, the absorption maximum was gradually blue-shifted when the amount of hexanol in mixed solvents was higher than 50%. Additionally, when PtBA133-b-PAzoMA19 was dissolved in hexanol, the population of cis isomers at the photostationary state reduced to 73.70%. This behavior can be explained by the structural change of PtBA133-b-PAzoMA19 with the addition of hexanol. When PtBA133-b-PAzoMA19 was dissolved in a neutral solvent, the random-coiled conformation of PtBA133-b-PAzoMA19 provided large free space for the conversion from trans isomers to cis isomers. By contrast, when PtBA133-b-PAzoMA19 was dissolved in a selective solvent, the confinement of azobenzene mesogens in the micelle core hindered photoisomerization, thereby causing incomplete trans-to-cis conversion. Furthermore, the addition of hexanol prolonged the time required to reach the photostationary state. To precisely determine the kinetics of photoisomerization, the progressive change in the UV spectra during UV irradiation was approximated by assuming the first-order kinetics, given by36 ln

A∞ − A0 = kt A∞ − At

(7)

24


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where At is the absorbance at the UV exposure time t, and k is the rate constant. Figure 7 presents a comparison of the plots of ln[(A∞ − A0)/(A∞ − At)] versus the UV irradiation time for PtBA133-b-PAzoMA19 in various mixed solvents. The rate constants of various conditions obtained by the slope of the linear fits are summarized in Table 5. The rate constant progressively decreased with an increase in the amount of hexanol in mixed solvents. The variation of the kinetics of photoisomerization with solvency was associated with the morphology of PtBA133-b-PAzoMA19 in mixed solvents. When dissolved in DMF, favorable compatibility between the solvent molecules and polymer chains resulted in the swelling of PtBA133-b-PAzoMA19 chains, thereby allowing conformational change without hindrance. When hexanol was added to DMF, the solvency of PtBA133-b-PAzoMA19 worsened, and PtBA133-b-PAzoMA19 chains were less swollen. The reduced free space for the trans-to-cis isomer conversion led to a decrease in the photoisomerization rate. Further increasing the amount of hexanol in mixed solvents caused micelle formation. The confined geometry, in which azobenzene moieties were segregated in the micelle core, restricted the occurrence of photoisomerization, resulting in a substantial decrease in the photoisomerization rate.

25


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Figure

7.

Plots

of

the

first-order

kinetics

Page 26 of 42

of

trans-to-cis

photoisomerization

for

PtBA133-b-PAzoMA19 in mixed DMF/hexanol solvents with different compositions.

Table 5. Rate constants of different PtBA-b-PAzoMA block copolymers in various mixed solvents obtained through UV spectra Rate constant (1/s)

a

100:0

90:10

DMF:hexanol 50:50 25:75

PtBA282-b-PAzoMA18

0.1341

0.1289

0.1270

0.0984

0.0551

0.0396

PtBA133-b-PAzoMA19

0.1169

0.1138

0.1066

0.0870

0.0533

0.0382

PtBA81-b-PAzoMA36

0.1175

0.1145

0.1123

0.0632

0.0446

--a

10:90

0:100

″--″ indicates insolubility of the system

As mentioned previously, when the amount of hexanol in mixed solvents was increased, the wavelength of the absorption maximum of PtBA133-b-PAzoMA19 in the UV spectra shifted to lower wavelengths. The shift in the wavelength of the absorption maximum resulted from the aggregation state of azobenzene moieties, in which the presence of H-aggregated, non-associated, and 26


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J-aggregated mesogens resulted in the occurrence of the maximum absorbance at wavelengths of 334, 360, and 384 nm, respectively.37 The contribution of each aggregation state can be obtained by the deconvolution of UV spectra according to the three absorption wavelengths (Figure S7). Figure 8 presents the fraction of each aggregation state in various systems. The solvent composition strongly affected the organization of mesogens. In the DMF-rich solution, PtBA133-b-PAzoMA19 exhibited a random-coiled conformation. The unconfined environment prevented the aggregation of azobenzene molecules. Therefore, the number of isolated mesogens was high. When the amount of hexanol in mixed solvents was increased, solvency worsened for the PAzoMA block, and PAzoMA segregated to reduce the repulsive interaction between PAzoMA and hexanol molecules. The segregated PAzoMA molecules in a compact space increased the possibility for azobenzene aggregation. Consequently, the population of the H-type aggregates increased with an increase in the amount of hexanol in mixed solvents.

Figure 8. Aggregation states of PtBA133-b-PAzoMA19 in mixed DMF/hexanol solvents with different compositions. 27


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Table 4 lists the parameters of the photoisomerization of different PtBA-b-PAzoMA block copolymers in various mixed solvents, which were obtained from UV spectra. Similar to PtBA133-b-PAzoMA19, solvency mainly controlled the photoresponsive behavior of various PtBA-b-PAzoMA block copolymers. When fPAzoMA increased, the solubility of PtBA-b-PAzoMA in mixed solvents decreased considerably, thereby causing the segregation of PAzoMA in the micelle core or clusters. Because fewer PtBA molecules were present in the micelle corona to prevent the PAzoMA block from direct contact with hexanol molecules, PAzoMA molecules were forced to pack in a more compact form. This in turn caused a reduction of the photoisomerization rate of PtBA-b-PAzoMA. Therefore, the photoisomerization rate of PtBA-b-PAzoMA decreased with an increase in fPAzoMA at the same solvent composition when the content of hexanol in mixed solvents was dominant.

Fluorescence behavior of PtBA-b-PAzoMA in mixed solvents Figure 9(a) shows the fluorescence emission of PtBA133-b-PAzoMA19 in mixed solvents with various compositions. Strong florescence emission at a wavelength of approximately 420 nm was observed upon excitation at 325 nm. The fluorescence intensity increased when the content of hexanol in the mixed solvent was 10%. Further increasing the amount of hexanol in the mixed solvent caused a reduction of fluorescence emission, and PtBA133-b-PAzoMA19 dissolved in pure hexanol exhibited the least fluorescence emission. Previous studies have reported that fluorescence 28


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emission from azobenzene molecules results from aggregation, which occurs when azobenzene mesogens are confined.12,13,19,20 The confinement of azobenzene-containing block copolymers can be achieved by mediating the formation of micelles. However, in this study, micelle formation by increasing the amount of hexanol in mixed solvents caused a decrease in fluorescence emission. Therefore, the concept of micellization-enhanced fluorescence cannot completely explain the mechanism of fluorescence emission. Another possible parameter affecting the fluorescence behavior of azobenzene derivatives in a solution is solvent polarity. In highly polar solvents, the vibrational fine structure, which contributes to fluorescence emission, is destroyed by strong solute– solvent interactions. This broadens the vibronic transition, resulting in fluorescence quenching.38 According to the literature, the normalized polarity (relative to water) of DMF and hexanol is 0.386 and 0.559, respectively.39 Adding hexanol to DMF increased the polarity of mixed solvents, which caused fluorescence loss. Hence, the results of our studies and previous studies10,17 agree with the concept of the solvent polarity effect. Conversely, some studies have reported that the fluorescence emission of azobenzene derivatives was enhanced when highly polar solvents were added to weakly polar solvents.12,19 Thus, the polarity of the solvent is not the key determinant affecting the fluorescence emission of azobenzene-containing block copolymers. As an alternative mechanism to the suggested mechanisms, we propose that the aggregation state of azobenzene mesogens is the major parameter contributing to this unusual fluorescence behavior. H-type aggregates exhibit a higher energy level at an excited state than J-type aggregates, and their energy release from the excited state to the ground state is mainly through vibrational relaxation, resulting in fluorescence 29


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loss.40,41 By contrast, nonassociated and J-aggregated mesogens release energy through fluorescence emission.12,42-44 In this study, when PtBA133-b-PAzoMA19 was dissolved in hexanol-rich mixed solvents, the population of H-aggregated mesogens dominated. Therefore, the presence of a high population of H-type aggregates in micelles quenched fluorescence. Conversely, although PtBA133-b-PAzoMA19 in DMF-rich mixed solvents did not form a confined geometry, the presence of a high population of nonassociated and J-aggregated mesogens enhanced fluorescence. Consequently, the fluorescence emission of PtBA133-b-PAzoMA19 in neutral solvents was more intense than that of PtBA133-b-PAzoMA19 in selective solvents. The concept of the aggregation-state effect can also be used to account for the fluorescence behavior of azobenzene-containing block copolymers in the literature.10,12,17,19

(a)

30


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(b) Figure 9. Fluorescence spectra (excitation at 325 nm) of (a) PtBA133-b-PAzoMA19 and (b) PtBA81-b-PAzoMA36 in mixed DMF/hexanol solvents with different compositions before UV irradiation.

Figure 9(b) presents the fluorescence emission of PtBA81-b-PAzoMA36 in mixed solvents with various compositions. Similar to PtBA133-b-PAzoMA19, in which the morphological transition from a random-coiled conformation to micelles weakened fluorescence, the transition from the random-coiled conformation to clusters in PtBA81-b-PAzoMA36 also caused a substantial decrease in fluorescence. Correspondingly, when 10% hexanol was added to DMF, the population of H-aggregates slightly decreased from 11.55% to 9.49%. Further addition of hexanol to DMF caused a monotonic increase in the population of H-aggregates, and the population of H-aggregates increased to nearly 50% for PtBA81-b-PAzoMA36 in the 10:90 DMF/hexanol solvent. The decrease in fluorescence emission was in good agreement with the increasing population of H-aggregates. 31


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This again confirms that the aggregation state of mesogens is closely related to the fluorescence behavior of these systems. Figure 10 shows the florescence emission of PtBA133-b-PAzoMA19 in various mixed solvents upon UV light irradiation. For all samples, UV irradiation enhanced fluorescence emission. When azobenzene mesogens were irradiated with UV light, rod-shaped trans isomers converted to bent-shaped cis isomers. The nonplanar bent-shaped conformation inhibited the photoinduced electron transfer of chromophores, which enhanced fluorescence emission.45,46 Moreover, nonplanar cis isomers disrupted the closely packed H-type aggregates. Consequently, radiative relaxation was induced, and fluorescence after UV irradiation was more intense than that before UV irradiation. Notably, when the fluorescence emission of PtBA133-b-PAzoMA19 in mixed solvents with the hexanol content of 50%, 75%, and 90% was compared, the fluorescence emission of PtBA133-b-PAzoMA19 in terms of the DMF/hexanol solvents was in the order of 50:50 DMF/hexanol > 25:75 DMF/hexanol > 10:90 DMF/hexanol. This was attributed to the content of cis isomers in various samples. After UV irradiation, PtBA133-b-PAzoMA19 in mixed solvents with a hexanol content of 50%, 75%, and 90% exhibited a population of cis isomers of 94.6%, 82.9%, and 74.0%, respectively. The higher population of cis isomers for PtBA133-b-PAzoMA19 in 50:50 DMF/hexanol resulted in more intense fluorescence emission.

32


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Langmuir

Figure 10. Fluorescence spectra (excitation at 325 nm) of PtBA133-b-PAzoMA19 in mixed DMF/hexanol solvents with different compositions after UV irradiation.

Conclusion We studied the effect of solvency on the phase behavior of azobenzene-containing block copolymers (PtBA-b-PAzoMA). When PtBA-b-PAzoMA was dissolved in DMF, DMF served as a good solvent for both blocks, thereby allowing the polymer chains to form a random-coiled conformation. By adding hexanol, a PtBA-selective solvent, to DMF, the repulsive interaction between the PAzoMA block and hexanol resulted in the self-assembly of PtBA-b-PAzoMA into spherical micelles composed of a PAzoMA core and a PtBA corona. Increasing the mole fraction of the PAzoMA block disallowed the full coverage of PtBA chains. Consequently, PtBA-b-PAzoMA formed clusters. The morphological change of PtBA-b-PAzoMA with solvency was correlated to its photoresponsive and fluorescence behaviors. Our results indicated that DMF-rich mixed solvents caused PtBA-b-PAzoMA chains to swell, which resulted in the formation of isolated and 33


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J-aggregated mesogens. Upon UV irradiation, an unconfined environment facilitated the conversion from trans isomers to cis isomers, allowing the rapid kinetics of photoisomerization and complete conversion. By contrast, when PtBA-b-PAzoMA was dissolved in hexanol-rich mixed solvents, the formation of micelles and clusters caused steric hindrance, leading to the aggregation of mesogens into an H-type association. Additionally, a confined geometry hampered trans-to-cis photoisomerization, thereby retarding the kinetics of photoisomerization. The morphology of PtBA-b-PAzoMA was also closely related to its fluorescence behavior. In contrast to the micellization-induced fluorescence emission reported in the literature, we observed fluorescence quenching when PtBA-b-PAzoMA formed micelles and clusters. This unusual fluorescence behavior was attributed to the aggregation state of mesogens. The dominant population of H-type aggregates in micelles and clusters when PtBA-b-PAzoMA was dissolved in hexanol-rich mixed solvents resulted in a nonradiative emissive process, whereas the increased number of nonassociated and J-aggregated mesogens when PtBA-b-PAzoMA was dissolved in DMF-rich mixed solvents activated the radiative process, leading to intense fluorescence. Upon UV irradiation, the presence of cis isomers inhibited the photoinduced electron transfer of chromophores. Cis isomers also enabled the unpacking of closely packed H-type aggregates. Consequently, fluorescence emission was enhanced considerably.

Acknowledgements This work was financially supported by the Ministry of Science and Technology (MOST) in Taiwan 34


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Langmuir

under Grant Nos. MOST 102-2221-E-006-018-MY3 and MOST 105-2628-E-006-009-MY3. Travel expenses were covered by the Neutron User Program (MOST 103-2739-M-213-001-MY3), managed by the National Synchrotron Radiation Research Center of Taiwan. The authors appreciate the support of the ACNS, ANSTO, Australia, in providing the Quokka SANS beam time (Proposal Nos. 4782 and 5707) and assistance during the SANS experiment.

Supporting Information. 1H NMR and infrared spectra of azobenzene-containing monomers; 1H NMR of the PtBA-Br macroinitiator; GPC of PtBA-b-PAzoMA, TEM images and UV spectra of PtBA-b-PAzoMA in mixed solvents; an example of the curve-fitting analysis applied to the UV spectrum of PtBA-b-PAzoMA in DMF

35


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Composed of Azobenzene-Containing Amphiphilic Diblock Copolymers under Irradiation at 436 nm. Eur. Polym. J. 2007, 43, 657–662. (9) Tsao,

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